Submitted to: American Association of Swine Practitioners Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 7, 1998
Publication Date: N/A
Technical Abstract: We have investigated the suitability of restriction fragment length polymorphism (RFLP) analysis for differentiating vaccine strain RespPRRS/Repro from field strains of PRRSV. We found that a differential RFLP test could distinguish the RespPRRS/Repro vaccine virus and its parent virus, VR-2332, from other North American strains. The test amplifies the PRRSV glycoprotein E gene (ORF 5) plus flanking sequences and identifies strains with restriction enzymes Mlu I, Hinc II and Sac II. In addition, we have sequenced the E gene for many recent PRRSV field isolates. A number of these 1996 isolates have RFLP patterns that are closely related to the RespPRRS/Repro RFLP pattern. We have designated the latter as 2-5-2 based on restriction sites for Mlu I, Hinc II and Sac II, respectively. The related or "intermediate" patterns of recent isolates had designations of 1-5-2, 2-1-2, 2-6-2 and 2-5-4. These patterns and their relationship to othe RespPRRS/Repro vaccine pattern will be discussed. Eleven virus isolations were made from experimentally infected pigs in order to gain additional insight into how the RFLP patterns change while PRRSV replicates in vivo. Pigs or gilts were infected with RespPRRS vaccine virus or field strain NADC-8. In 3 experiments, a second pig was contact-infected at a later time and the PRRSV was recovered from the contact pig. In all 11 experimental infections, the RFLP patterns were unchanged. The number of nucleotide differences ranged from unchanged after 2 weeks of replication in a single pig to 5 nucleotide differences after 13 weeks of in vivo replication and transmission of virus between pigs. These experiments give some baseline values for the rate of genetic change in the E gene while PRRSV replicates in vivo.